Heretofore, various optical scanning devices such as laser printers and copying machines for displaying and recording an image by scanning with a laser beam have been known. This type of optical scanning device is made so that a laser beam from a laser diode is collimated by a collimator lens, is scanned by being reflected off a rotating polygon mirror, and is then stopped down by an f-.theta. lens and is made incident onto a photoconductive surface of a photosensitive drum. In this manner, the scanning of a small spot of light onto a recording medium is achieved.
However, with this type of optical scanning device it becomes necessary to employ an overall lens system having a high numerical aperture in order to achieve a sufficiently high luminance on the photosensitive drum. Further, the numerical aperture of the collimator lens is large compared to that of a general f-.theta. lens, thereby making the aberrations caused by the collimator lens more troublesome. Therefore, it becomes essential to constrain the optical aberrations in the collimator lens in order to make the spot on the photosensitive drum as small as desired.
A collimator lens as described above is disclosed, for example, in Japanese Laid-Open Patent Application H08-114768. However, when the collimator lens is placed in proximity to a light source (such as a laser diode), the temperature of the disclosed collimator lens increases due to the heat from the light source. Hence there is a problem in that the heating of the collimator lens causes the optical aberrations to increase. Therefore, it becomes essential to increase the back focal distance of the collimator lens so that the distance between the collimator lens and the light source is larger in order to thermally isolate the collimator lens from the heat of the light source. Such a collimator lens, having a three lens composition, is disclosed in, for example, Japanese Laid-Open Patent Application H08-114767. However, with the collimator lens described therein, there is a problem in that the optical aberrations increase when the light source (such as a laser diode) or the collimator lens experiences mis-positioning in a direction parallel to the optical axis (hereinafter termed slippage). Further, when either the light source or the collimator lens experiences mis-positioning normal to the optical axis (hereinafter termed de-centering), there is the problem that the position of light formation on the photosensitive drum changes.